Chronic orthostatic intolerance takes the form of postural tachycardia syndrome (POTS) in many patients. One type of Low flow POTS (LFP) has increased vasoconstriction associated with increased angiotensin-II (Ang- II), reduced nitric oxide (NO), and increased reactive oxygen species (ROS). We hypothesize that LFP is due to increased central sympathetic activity or neurovascular sympathetic transduction caused by Ang-II binding to angiotensin type 1 receptors (AT1R) activating NADPH oxidase or Xanthine oxidase (XO) to produce ROS. ROS include superoxide which scavenges NO to produce peroxynitrite, and H2O2 which exerts important vasoactive and sympathetic effects. The proposal comprises two parts: the first explores causes of increased Ang-II; the second examines effects of Ang-II on oxidative stress, sympathetic activity and neurovascular transduction. Skin will continue as a surrogate tissue to explore NO, Ang-II and ROS. Studies will also explore connections among muscle sympathetic nerve activity (MSNA), peripheral blood flow, and arterial BP as well as potential treatments. The hypothesis will be tested by comparing patients with LFP (N=30), to patients with normal flow POTS (N=30), and to healthy volunteers (N=30) to answer the following questions: 1) Is cutaneous microvascular NO deficiency in LFP caused by Ang-II/oxidase induced oxidative stress? Experiments use intradermal microdialysis probes, laser Doppler flowmetry, and the NO-dependent local heating response to measure Ang-II and Ang-(1-7) in the skin, and to examine the effects of NADPH oxidase/XO blockade with apocynin/allopurinol, and superoxide/H2O2 reduction with tempol/ebselen. Intradermal ROS are measured using intracatheter reactions of superoxide and peroxynitrite while H2O2 is assessed using fluorescent spectrophotometry. We will determine if sodium ascorbate and losartan improve cutaneous NO and will correlate skin responses with systemic responses to intravenous ascorbic acid and oral losartan in later experiments. 2) Do cutaneous angiotensin-II receptors and NOS isoforms contribute to LFP? Skin punch biopsies will determine NOS-isoform, AT1R and AT2R, and ACE2 and ACE mRNA expression and protein content. 3) How do central sympathetic activation and neurovascular transduction contribute to vasoconstriction? Peroneal microneurography, popliteal artery ultrasound, and spontaneous BP oscillations will be used to assess MSNA, baroreflex activity, and the neurovascular transduction of MSNA to peripheral resistance. 4) Can intravenous infusion of the antioxidant ascorbic acid restore sympathetic activity, baroreflex function and orthostatic tolerance in LFP? The central and peripheral neurovascular effects will be examined. 5) Can chronic AT1R blockade with losartan restore sympathetic activity, baroreflex function and orthostatic tolerance? A double blind, placebo controlled study of chronic oral losartan treatment in LFP will be performed with reassessment of its effects on MSNA, baroreflex, and neurovascular transduction.